1. Field of the Invention
The present invention relates to a fiber optic cable comprising a plurality of optical fibers bundled therein and to an optical transmission system in which the fiber optic cable is installed as an optical transmission line.
2. Related Background Arts
In optical communications performed at high speed such as tens of Gb/s by means of a fiber optic cable, the waveform degradation of signal light due to the chromatic dispersion inherent in an optical fiber becomes a problem. The higher the bit rates of signal light, the smaller absolute value of the accumulation of the chromatic dispersions of an optical fiber is required between the transmitting end and the receiving end. Therefore, dispersion-shifted optical fibers having a zero dispersion wavelength in the 1.55 xcexcm band, in which signal light exists, are used.
However, even in the case of a fiber optic cable in which the dispersion-shifted optical fibers are bundled, the absolute value of the chromatic dispersion of the optical fibers increases depending on temperature fluctuations in the environment where the fiber optic cable is installed. Consequently, there are cases where the level of waveform degradation of signal light becomes too high to ignore for transmitting signal light at high bit rates.
It is reported in Literature 1: Kwang S. Kin, et al., xe2x80x9cTemperature Dependence of Chromatic Dispersion in Dispersion-shifted Fibers: Experiment and Analysisxe2x80x9d, J. Appl. Phys., Vol.73, No.5, pp.2069-2074 (1993) that the variation quantity per unit temperature of the chromatic dispersion of a dispersion-shifted optical fiber is about xe2x88x920.0023 to xe2x88x920.0015 psxc2x7nmxe2x88x921xc2x7kmxe2x88x921xc2x7Kxe2x88x921. With such an optical fiber, signal light can be transmitted only about 200 km distance, when the bit rates are 40 Gb/s.
On the other hand, the technology described in Literature 2: K. Yonenaga, et al., xe2x80x9cTemperature-independent 80 Gbit/s OTDM Transmission Experiment Using Zero-dispersion-flattened Transmission Linexe2x80x9d, ECOC ""99 (1999) is intended for solving the problem of fluctuations in accumulated values of the chromatic dispersions due to temperature fluctuations. By coupling two kinds of optical fibers, which are different from each other in temperature dependence of the chromatic dispersion, the temperature dependence of accumulated chromatic dispersions is reduced to achieve a long-distance signal light transmission at high bit rates.
The technology described in Literature 2 is based on the assumption that the temperature is constant over the full length of a transmission line and no consideration is given to the environment where a fiber optic cable is actually to be installed. However, fiber optic cables are generally installed under a natural environment, such as in the ground or the ocean floor. Consequently, the temperatures of the two kinds of optical fibers thus connected occasionally differ from one another due not only due to temperature fluctuations because of the temporal changes such as seasons, day or night, but also to temperature differences because of regional changes. In such cases, the temperature dependence of the accumulated value of the chromatic dispersions will not be reduced, and accordingly long-distance signal light transmission cannot be performed at high bit rates.
An object of the present invention is to provide a fiber optic cable that can perform long-distance signal light transmission at high bit rates not withstanding any temporal temperature change or regional temperature difference.
Another object of the present invention is to provide an optical transmission system in which signal light can be transmitted at high bit rates over a long distance, from a transmitting station to a receiving station, notwithstanding a temporal temperature change or a regional temperature difference to the fiber optic cables installed between the transmitting station (or a relay station) and the receiving station (or a relay station).
To achieve the above-mentioned objects, herein provided is a fiber optic cable that comprises a plurality of optical fibers bundled therein which have an absolute value of 0.001 psxc2x7nmxe2x88x921xc2x7kmxe2x88x921xc2x7Kxe2x88x921 or less of chromatic dispersion variation quantity per unit temperature at a wavelength of 1550 nm. Also provided is an optical transmission system in which such a fiber optic cable is installed as an optical transmission line.
The above and further objects and novel features of the invention will be more fully clarified from the following detailed description when the same is read in connection with the accompanying drawings. It is to be expressly understood, however, that the drawings are for the purpose of illustration only and are not intended as a definition of the limits of the invention.